1. Field of the Invention
The present invention relates to a die for hot die face cutting of thermoplastic polymers.
More particulaly, the present invention relates to a die for hot die face pelletization of thermoplastic polymers by cutting the polymeric monofilaments directly on the front of the die.
2. Description of the Prior Art
Bibliographic and patent literature report different types of devices for hot die face pelletization of thermoplastic materials.
Such devices essentially comprise:
(a) a die consisting of a heated bored body; PA0 (b) a cutting device including a rotating driving shaft provided with a blade mounting disk and a system to position the blades against the outer face of the die; PA0 (c) a cutting chamber collecting the pellets, and PA0 (d) a system for feeding and distributing a fluid, generally water controlled by a thermostat, necessary for cooling and carrying the cut pellets outside the cutting chamber.
The thermoplastic molten material is extruded through the die holes; the monofilaments coming out of the die are cut by the rotating blades and the pellets thus obtained are cooled and removed by means of the thermostated fluid.
The thermostated fluid is also intended to prevent adhesion of the pellets on the blades, on the die surface and on the other parts of the equipment and to prevent agglomeration of the pellets themselves.
The cutting operation can be performed by keeping the die completely immersed in the fluid (underwater pelletizing method) or by spraying the die with the fluid (water spray pelletizing method).
The main drawback of such known devices is that, in particular situations, the thermostated fluid cools the die at a rate greater than that at which heat can be restored by the thermoregulating system. As a consequence, the thermoplastic material solidifies (freezes) inside the die holes, and therefore, the extrusion process stops due to obstruction of the holes.
To overcome this drawback in the cases where the thermoregulating system of the die is unable to restore the heat losses, attempts where made to protect the die from a very intense cooling. Many methods were proposed for this purpose, such as: to reduce heat losses of the die, in correspondence to the surface in contact with the fluid, by inserting a layer of insulating material under the bored plate of the die or by externally coating the same; to thermally protect the molten polymer in die channels by internally coating each channel with insulating materials, etc.
All these methods, however, used alone or in combination, do not eliminate the problem of freezing of the polymer in the holes in a satisfactory way, especially during the device start-up, causing more or less serious drawbacks depending on the polymer type, the hole sizes, the polymer flow-rate and the processing conditions. Therefore, the performance is negatively affected with the following possible limitations: impossibility of reducing the polymer flow rate for each hole below critical values, impossibility of cooling the cutting area using fluid at a temperature below critical values, and so on.